The invention relates to a method and apparatus for accurately placing a surface mounting device (SMD) on predetermined pads of a printed circuit board with the aid of machine vision.
Conventional pick-and-place machines carry surface mount devices with a vacuum nozzle. Displacement and rotation errors in the position and the orientation of the SMD relative to the nozzle limit positioning accuracy. That is to say, if the SMD is misaligned with the nozzle, and the pick-and-place machine places the nozzle in a desired nozzle position, the SMD will be misaligned with the desired SMD position. Use of different nozzles for different SMD sizes improves accuracy but sacrifices system flexibility. The number of fixtures required will increase with the increase in sizes of SMDs. The frequency of replacing fixtures will also increase, system efficiency will decrease, costs will rise and maintenance will be more difficult. Moreover, such nozzles orient the SMD body rather than the leads. Offset between the center line of an SMD body and the center line of the leads may be as large as seven mil (1 mil=1/1000 in.) therefore, use of different fixtures provides only limited improvement in accuracy.